1. Field of the Invention
The present invention relates to an electroluminescent display (EL), and more particularly, to a bi-directional emission-type EL display.
2. Description of the Related Art
EL displays are active light-emitting display devices that have numerous advantages over cathode ray tubes (CRTs) or liquid crystal displays (LCDs). EL displays can provide a wider viewing angle, higher contrast, faster response speed, lighter weight, smaller size, thinner thickness, and lower power consumption. There is an increasing interest on developing EL displays as next generation displays. EL displays are classified into inorganic and organic EL displays, depending on whether the light-emitting layer is made of organic or inorganic material.
FIG. 1 shows a conventional bi-directional emission-type EL display having a structure similar to that disclosed in U.S. Pat. No. 6,469,437, which hereby is incorporated by reference. The EL display has a sequentially stacked structure including an anode 20, an intermediate layer 30, and a cathode 40 on a transparent substrate 10. As shown in FIG. 1, the intermediate layer 30 includes a hole injection layer 31, a hole transport layer 32, a light-emitting layer 33, an electron transport layer 34, and an electron injection layer 35. Except for the light-emitting layer 33, the other layers 31,32,34,35 may be omitted, if necessary. Since the EL display is a bidirectional emission-type display, the anode 20 and the cathode 40 are made of a transparent material, such as indium tin oxide (ITO). Light emitted from the light-emitting layer 33 is discharged towards both the anode 20 and the cathode 40. However, since light emitted from the light-emitting layer 33 is not reflected by the anode 20 or the cathode 40, it is impossible to accomplish the enhancement of light emission characteristics by constructive interference due to a resonance effect of light emitted from the light-emitting layer 33. Weak or low amplitude light emitted from the light-emitting layer 33 does not pass through the anode 20 and the cathode 40, and becomes trapped between the anode 20 and cathode 40. Energy from the trapped light is absorbed in the layers interposed between the anode 20 and the cathode 40. Accordingly, light efficiency of the conventional EL display is reduced.